As a subclass of commercially available polymers, polyurethane elastomers have several properties whose advantages confer unique benefits on these products. Typically, polyurethanes show high abrasion resistance with high load bearing, excellent cut and tear resistance, high hardness, resistance to ozone degradation, yet are pourable and castable. Compared to metals, polyurethanes are lighter in weight, less noisy in use, show better wear and excellent corrosion resistance while being capable of cheap fabrication. Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibit good elastomeric memory. Polyurethanes find use in such diverse products as aircraft hitches, bushings, cams, gaskets, gravure rolls, star wheels, washers, scraper blades, impellers gears, and drive wheels.
Part of the utility of polyurethanes derives from their enormous diversity of properties resulting from a relatively limited number of reactants. Typically, polyurethanes are prepared on site by curing urethane prepolymers, which are adducts of polyisocyanates and polyhydric compounds. A large class of such prepolymers are approximately 2:1 adducts of a diisocyanate, OCN--Y--NCO, and a diol. Another class of prepolymers, especially pertinent to this application, result from the reaction of diisocyanates with a limited amount of water to give as prepolymers principally biurets with smaller amounts of higher condensation products according to the reaction. ##STR1## Although Y is susceptible of great variety, usually being a divalent alkyl, cyclohexyl, or aromatic radical, in fact the most available urethane prepolymers are made from toluene-2,4-diisocyanate (TDI) or methylene-4,4'-diphenylisocyanate (MDI), although the biurets show a somewhat greater structural variation.
The polyurethane elastomers and polyureas are formed by curing the urethane prepolymer or biuret, respectively. Curing is the reaction of the terminal isocyanate groups of the prepolymer or biuret with active hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, crosslinking. Diols, especially alkylene diols, are the most common curing agents, especially for MDI-based urethane prepolymers, and representing such diols with the structure HO--X--OH, where X is an organic moiety, most usually an alkylene group, the resulting polymer has as its repeating unit, EQU (--Y--NHCO.sub.2 XO.sub.2 CNH--Y--NHCO.sub.2 X--O--CONH--)
Where a triol or a higher polyhydric alcohol is used crosslinking occurs to afford a nonlinear polymer. Where biurets are the prepolymer crosslinking occurs even with diols, since the biuret itself is at least trifunctional.
Although other polyfunctional chemicals, especially diamines, are theoretically suitable as a curing agent, with but a few exceptions none have ahieved commercial importance. The major exception is 4,4'-methylene-di-ortho-chloroaniline, usually referred to as MOCA, a curing agent which is both a chain extender and a crosslinker. TDI-based urethane prepolymers typically are cured with as MOCA, and the resulting products account for perhaps most of the polyurethane elastomer market. One reason that polyhydric alcohols generally have gained acceptance as curing agents is that their reaction with urethane prepolymers is sufficiently fast to be convenient, but not so fast as to make it difficult to work with the resulting polymer. In casting polymers it is desirable that the set-up time be reasonably short, yet long enough for the material to be cast into molds. This property is conventionally referred to as pot life. Generally speaking, diamines react with prepolymers, and especially MDI-based prepolymers, so quickly that they are not usable as curing agents. However, primary aromatic diamines with electronegative groups in the aromatic ring, or with alkyl groups ortho to the amino moiety, exhibit sufficiently decreased reactivities with some prepolymers as to afford a desirable pot life, hence the use of, for example, MOCA as a curing agent for TDI-based urethane prepolymers. However, MOCA and other of the aforementioned diamines still remain too reactive to be used, for example, with MDI-based urethane prepolymers.
Previously only primary aromatic diamines seem to have been used as curing agents. Presumably this is because secondary diamines were expected to have an unacceptably long pot life, and because they could act only as chain extenders with urethane prepolymers in contrast to the crosslinking capabilities of primary diamines. Recently, however, we have found that certain N,N'-dialkyl-4,4'-methylene-dianilines are generally effective curing agents for a broad range of urethane prepolymers. The resulting polyurethanes often have the advantage of being thermoplastic rather than thermosetting, thereby making them especially useful as coatings, adhesives, and sealants. We also have found that such aromatic alkyl diamines are effective curing agents for biurets which, because of the latter's trifunctionality (and higher functionality), are crosslinked by these diamines to afford polyurea resins with quite desirable properties.
Polyurethanes find extensive application as coatings and adhesives. Among the properties of polyurethane particularly desirable in the coating art are their chemical resistance, light stability, flexibility, toughness, weatherability, moisture resistance, abrasion resistance, gloss and color retention, and impact resistance. We have found that biuret prepolymers cured with the secondary amines we previously described afford resins which are particularly suitable as coatings, and this application is directed toward that use.